Many power supplies are designed to provide both a positive and negative output polarity of output current and a unipolar output voltage. An example of such a power supply could be a switching power supply employing synchronous rectifiers. When the output voltage and the output current are both positive, the power supply is said to be operating in “quadrant one.” Quadrant one operation is shown in FIG. 1A. FIG. 1A shows a graphical illustration 100 in which the horizontal axis 102 represents output current and the vertical axis 104 represents output voltage. The trace 110 illustrates a positive output voltage and a positive output current. If the net energy flow is from the primary (input) side of the power supply to the secondary (output) side of the power supply, the power supply is said to be “sourcing” power.
It is also desirable to have the ability to reduce the output voltage when a lower voltage output is desired. In order to rapidly reduce the output voltage of the power supply, typically a circuit is coupled to the secondary side of the power supply and is employed to allow energy to dissipate from the capacitances, or possibly a battery or other energy source coupled to the secondary side of the power supply. The circuit is frequently implemented as a transistor and related circuitry connected across the output side of the power supply. When a reduction in output voltage is desired, the circuit provides a path through which to discharge the secondary side output capacitors or battery until the desired voltage level is reached. This is a state of operation in which the voltage is positive, but the output current is negative. When a power supply is operating in such a condition, it is said to be operating in “quadrant two.” In a bidirectional switching power supply, if the net energy flow is from the secondary (output) side of the power supply to the primary (input) side of the power supply, the power supply is said to be “sinking” power and the circuit is referred to as a “current sinking” circuit.” Such is the case, for example, when either the output capacitors or a battery coupled to the secondary side of the power supply are discharged.
Operation in quadrant two may be momentary, such as when discharging an output capacitor, or continuous, such as when discharging a battery.
Quadrant two operation is shown in FIG. 1B. FIG. 1B shows a graphical illustration 105 in which the horizontal axis 102 represents output current and the vertical axis 104 represents output voltage. The trace 120 illustrates a positive output voltage and a negative output current. When operating in quadrant two, a power supply is said to be “sinking” current from the secondary side of the power supply.
In the past, a current sinking circuit has been added to the secondary side of a power supply that typically operates only in quadrant one to allow for rapid discharge of the output and load capacitances associated with the output circuitry. Further, in some applications, the current sinking circuit acts as a steady state “load” to sink current. This provides at least some degree of quadrant two operation for a power supply designed to operate in quadrant one.
Prior art solutions that achieve quadrant two operation by the addition of a current sinking circuit on the secondary side of the power supply may have difficulty achieving a smooth transition from sourcing to sinking current. In quadrant one operation, a voltage control loop associated with the power supply operates through a switching supply pulse width modulator to produce positive output current. In quadrant two operation, the same voltage control loop must operate the current sinking circuit while the pulse width modulator is off. As a result, a smooth transition from sourcing current to sinking current can be very difficult to achieve because of the very different loop dynamics involved.
In addition, other difficulties can be encountered when controlling a current sinking circuit located on the secondary side of a power supply when the output voltage is low. This causes any transistors associated with the current sinking circuit to operate near saturation. When the transistors in the current sinking circuit saturate, non-linear control loop behavior can cause anomalies, glitches or oscillation on the power supply output. The design of a control loop to overcome these problems can be very difficult to achieve and can be very complex.
Further, when placing current sinking circuitry across the secondary side of the power supply, the voltage applied to the current sinking circuit is not constant, requiring the use of transistors, which must withstand both high current and high voltage. This increases the cost of the current sinking circuit and makes a non-optimal design.
Therefore, it would be desirable to have a way to transfer energy from the output side of a power supply at any output voltage, while maintaining a steady voltage across the current sinking circuitry.